1. Field of the Invention
The present invention relates to color image data compressing and reproducing methods which is appropriate when used in case primary color image data are stored and transmitted after they have been converted into binary image data.
2. Description of the Prior Art
An output device such as a display device or a printing device outputs binary, i.e., black and white image data which have been converted from an original image. The dither method is well known in the art as a method of expressing a pseudo-halftone by that output device.
This dither method is one kind of area gradation, by which the halftone image is expressed by varying the number of dots to be recorded in a constant area (or matrix) to record a portion corresponding to one picture element of a document in one dot, as shown in FIG. 13. As a result, there are obtained binary output data, as shown. These output data express a halftone image in pseudo-binary values of white and black.
Now, a number of methods are known for coding the halftone white and black image thus obtained. For coding the color halftone image, a main method is to separately code the three halftone images which are formed from the respective primary color signals R, G and B.
An example of the prior art, in which a block coding or one method of coding the halftone image is applied to a halftone color image, is shown in FIG. 14.
The block coding method is an information nonretained type image coding. Therefore, an original halftone image is divided into block images of totally 16 picture elements of 4.times.4 picture elements, and for each block an average picture element level P.sub.0 is calculated to determine a picture element level average P.sub.1 lower than P.sub.0 and a picture element level average P.sub.2 higher than P.sub.0.
Next, the resolution information is prepared by setting picture elements lower than P.sub.0 at "0" and picture elements higher than P.sub.0 at "1". In the coding method, moreover, the information compression is conducted for that resolution information and the gradation information of P.sub.1 and P.sub.2.
In case the original block image is shown in FIG. 14(a), for example, the average value P.sub.0 of the image levels of all the blocks is expressed by P.sub.0 =7 (as shown in FIG. 14(b)). Likewise, the average P.sub.2 of the image levels (of totally 6 picture elements) higher than that average P.sub.0 is expressed by P.sub.2 =12. On the other hand, the average P.sub.1 of the image levels (of totally 10 picture elements) lower than the average P.sub.0 is expressed by P.sub.1 =4.
As a result, the resolution information obtained by comparing the image level of the original block image with the average P.sub.0 is shown in FIG. 14(c). This resolution information (i.e., the information of "1" and "0") and the information on the averages P.sub.1 and P.sub.2, i.e., the gradation information are stored and then transmitted.
The image decoding is conducted by applying the gradation information P.sub.1 to the picture elements of the resolution information "0" and the gradation information P.sub.2 to the picture elements of the resolution information "1". As a result, a decoded image is obtained, as shown in FIG. 14(d).
This block coding method is applied to the block coding of each color halftone image.
In case the color halftone image is to be block-coded, as shown in FIG. 15, three primary color images R, G and B are individually block-coded by the aforementioned method, and the coded information is once stored in memory means until it is transmitted to destinations through a telephone or leased circuit.
At a reception side, the transmitted information is individually block-coded to reproduce a color halftone image.
Incidentally, men have the following visual characteristics in relation to the image discrimination:
(1) Generally speaking, men have a low gradation discriminating capability in a region of large picture element level variations but a high one in a region of small picture element level variations; and
(2) In a color image, men are sensitive to variations in a luminance level but insensitive to those in a chromaticity level.
In the example of the prior art shown in FIG. 15, on the contrary, the aforementioned visual characteristics are not taken into consideration because the primary color signals of the three R, G and B channels are block-coded as they are. As a result, the image data are liable to be coded, even if they need not be intrinsically coded, to raise a serious obstruction against the coding efficiency.